Optical glass

ABSTRACT

The optical glass of the present invention comprising SiO 2 , alkali metal oxides, P 2  O 5  and Nb 2  O 5  has optical properties of 1.70 or more in refractive index nd and 30 or less in Abbe&#39;s number νd and is superior in devitrification resistance and stability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical glass and more particularly to an optical glass of high refractive index and high dispersion having a refractive index nd of 1.70 or more and an Abbe's number νd of 30 or less.

2. Description of Prior Art

As such an optical glass, there are disclosed, for example, a glass comprising, as main components, P₂ O₅, Na₂ O, TiO₂ and WO₃, in Japanese Patent Application Kokai (Laid-Open) No. 17409/1974, a glass consisting of a P₂ O₅ -R₂ O (R is a mono-valent metal)-ZnO-Nb₂ O₅ type glass, in Japanese Pat. Publication No. 40094/1981, and a glass in which BPO₄ and/or Al(PO₃)₃ is incorporated into a SiO₂ -TiO₂ -Nb₂ O₅ -R₂ O (R is a mono-valent metal) type glass, in Japanese Pat. Publication No. 54247/1985.

However, the glasses disclosed in Japanese Pat. Application Kokai (Laid-Open) No. 17409/1974 and Japanese Pat. Publication No. 40094/1981 have low devitrification resistance. The glass disclosed in Japanese Pat. Publication No. 54247/1985, when the refractive index nd is increased to 1.70 or more, has a high devitrification tendency in the vicinity of the softening temperature and accordingly is devitrified when the glass block is heated and softened, making the press molding difficult.

Hence, an object of the present invention is to provide a high refractive index and high dispersion optical glass which has a refractive index nd of 1.70 or more and an Abbe's number νd of 30 or less and which is highly resistant to devitrification and stable.

Other objects of the present invention are apparent from the following descriptions.

SUMMARY OF THE INVENTION

The present invention made study in order to achieve the above object. As a result, it was found that the optical glass of the below given composition comprising SiO₂, alkali metal oxides, P₂ O₅ and Nb₂ O₅ has optical properties of 1.70 or more in refractive index nd and 30 or less in Abbe's number νd and is superior in devitrification resistance and stability. This finding has led to the completion of the present invention.

The present invention resides in an optical glass of high refractive index and high dispersion comprising

    ______________________________________                                         SiO.sub.2             5-30%    by weight                                       GeO.sub.2             0-25%    by weight                                       the sum of SiO.sub.2 and GeO.sub.2                                                                   5-30%    by weight                                       Na.sub.2 O            0-19%    by weight                                       K.sub.2 O             0-19%    by weight                                       the sum of Na.sub.2 O and K.sub.2 O                                                                  10-19%   by weight                                       P.sub.2 O.sub.5       10-35%   by weight                                       Nb.sub.2 O.sub.5      18-49%   by weight                                       TiO.sub.2             0-14%    by weight                                       Ta.sub.2 O.sub.5      0-20%    by weight                                       ZrO.sub.2             0-10%    by weight                                       WO.sub.3              0-5%     by weight                                       PbO                   0-20%    by weight                                       ZnO                   0-10%    by weight                                       Bi.sub.2 O.sub.3      0-20%    by weight                                       the sum of Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, ZrO.sub.2,                                            18-49%.3,                                                                               by weight                                       PbO, ZnO and Bi.sub.2 O.sub.3                                                  B.sub.2 O.sub.3       0-15%    by weight                                       Al.sub.2 O.sub.3      0-5%     by weight                                       and further comprising                                                         As.sub.2 O.sub.3      0-3%     by weight                                                            based on the total                                                            amount of the above                                                            components.                                                ______________________________________                                    

DETAILED DESCRIPTION OF THE INVENTION

The role and amount of each component constituting the optical glass of the present invention are described below.

In the optical glass of the present invention, SiO₂ is a component which is effective for increasing the viscosity of the resulting glass and reducing the devitrification tendency and is suitable for increasing the chemical durability and reducing the expansion coefficient. When the SiO₂ amount is less than 5% by weight, the above effects are not obtained. When the amount is more than 30% by weight, the resulting glass has no optical properties as desired. Therefore, the SiO₂ amount is limited to a range of 5-30% by weight, preferably 5-26% by weight. GeO₂ is a component which is effective for increasing the refractive index of the resulting glass. When the GeO₂ amount is more than 25% by weight, the resulting glass is highly colored and has a high devitrification tendency at low temperatures. Therefore, the GeO₂ amount is limited to a range of 0-25% by weight, preferably 0-18% by weight. When the total amount of SiO₂ and GeO₂ is more than 30% by weight, the resulting glass has no optical properties as desired. Therefore, the total amount of SiO₂ and GeO₂ is limited to a range of 5-30% by weight, preferably 5-26% by weight.

Na₂ O and K₂ O are each a component which is important for improving the meltability of the resulting glass and controlling the devitrification tendency. When the respective amounts of Na₂ O and K₂ O are more than 19% by weight, the resulting glass has no optical properties as desired. Accordingly, the Na₂ O amount is limited to a range of 0-19% by weight, preferably 5-19% by weight, and the K₂ O amount is limited to a range of 0-19% by weight, preferably 5-18% by weight. When the total amount of Na₂ O and K₂ O is less than 10% by weight, their effect is very low. When the total amount is more than 19% by weight, the resulting glass has no optical properties as desired. Therefore, the total amount of Na₂ O and K₂ O is limited to a range of 10-19% by weight, preferably 13-19% by weight.

P₂ O₅ is a component which is effective for obtaining a glass of high dispersion. When the P₂ O₅ amount is less than 10% by weight, the resulting glass has no optical properties as desired. When the amount is more than 35% by weight, the resulting glass has a high devitrification tendency and low chemical durability. Therefore, the P₂ O₅ amount is limited to a range of 10-35% by weight, preferably 10-30% by weight.

Nb₂ O₅ is a component which is effective for obtaining a glass of high refractive index and high dispersion. When the Nb₂ O₅ amount is less than 18% by weight, the resulting glass has no optical properties as desired. When the amount is more than 49% by weight, the resulting glass has a high devitrification tendency. Therefore, the Nb₂ O₅ amount is limited to a range of 18-49% by weight, preferably 22-42% by weight.

TiO₂ is a component which is effective for obtaining a glass of high refractive index and high dispersion. When the TiO₂ amount is more than 14% by weight, the resulting glass has a high devitrification tendency. Therefore, the TiO₂ amount is limited to a range of 0-14% by weight, preferably 4-14% by weight. Ta₂ O₅, ZrO₂, WO₃, PbO, ZnO and Bi₂ O₃ may be added to replace part of Nb₂ O₅ or TiO₂, whereby the optical constants of the resulting glass can be adjusted. However, when the amounts of Ta₂ O₅, ZrO₂, WO₃. PbO, ZnO and Bi₂ O₃ are more than 20% by weight, 10% by weight, 5% by weight, 20% by weight, 10% by weight and 20% by weight, respectively, the resulting glass has a high devitrification tendency and low dispersion and no desired glass can be obtained. Therefore, the Ta₂ O₅ amount is limited to a range of 0-20% by weight, preferably 0-15% by weight; the ZrO₂ amount to a range of 0-10% by weight, preferably 0-5% by weight; the WO₃ amount to a range of 0-5% by weight, preferably 0-3% by weight; the PbO amount to a range of 0-20% by weight, preferably 0-15% by weight; the ZnO amount to a range of 0-10% by weight, preferably 0-5% by weight; and the Bi₂ O₃ amount to a range of 0-20% by weight, preferably 0-15% by weight. Further, the total amount of Nb₂ O₅, Ta₂ O₅, ZrO₂, WO₃, PbO, ZnO and Bi₂ O₃ is limited to 18-49% by weight, preferably 22-44% by weight.

B₂ O₃ is a component which is effective for obtaining a glass of improved meltability and relatively high dispersion. When the B₂ O₃ amount is more than 15% by weight, the resulting glass has a high devitrification tendency. Therefore, the B₂ O₃ amount is limited to a range of 0-15% by weight, preferably 3-15% by weight.

Al₂ O₃ is a component which is effective for obtaining a glass of high dispersion and improved chemical durability. When the Al₂ O₃ amount is more than 5% by weight, the resulting glass has a high devitrification tendency. Accordingly, the Al₂ O₃ is limited to a range of 0-5% by weight, preferably 0-3% by weight.

As₂ O₃ is a component which is effective as an achromatic agent when the Nb₂ O₅ and TiO₂ amount is high. When the As₂ O₃ amount is more than 3% by weight, the resulting glass has a high devitrification tendency. Therefore, the As₂ O₃ amount is limited to a range of 0-3% by weight, preferably 0-1% by weight. Incidentally, the As₂ O₃ amount is determined based on the total weight of all the glass components other than As₂ O₃.

As is ordinarily done, small amounts of Sb₂ O₃, F, etc. may also be added for the purposes of achromatization, deaeration, etc.

EXAMPLES

The present invention is described in more detail below by way of Examples.

Total 19 kinds of optical glasses of high refractive index and high dispersion all according to the present invention were produced as follows.

In the optical glasses of Examples 1-19, the following substances were used as raw materials for the components. That is, as the raw material for P₂ O₅, there was used an aqueous orthophosphoric acid (H₃ PO₄) solution or a phosphate such as potassium metaphosphate or the like; and as the raw materials for other components, there was used an oxide, a carbonate, a phosphate, a nitrate or the like corresponding to each component. For example, in the optical glass of Example 5 comprising 5.0% by weight of SiO₂, 18.0% by weight of GeO₂, 5.0% by weight of Na₂ O, 10.0% by weight of K₂ O, 17.4% by weight of P₂ O₅, 29.3% by weight of Nb₂ O₅, 6.7% by weight of TiO₂ and 8.6% by weight of B₂ O₃ (the total amount of the above components is 100% by weight) and further comprising, as a deaerating agent, 0.2% by weight, based on the above components, of As₂ O₃, there were used, as raw materials, respective oxides for SiO₂, GeO₂, Nb₂ O₅, TiO₂ and As₂ O₃, respective nitrates for Na₂ O and K₂ O, an aqueous orthophosphoric acid (H₃ PO₄) solution for P₂ O₅, and boric acid (H₃ BO₃) for B₂ O₃. The optical glasses of Examples 1-19 were produced by mixing the above mentioned raw materials in such proportions as to give a composition as shown in Table 1 or 2, melting the mixture in a platinum crucible at 1,200°-1,400° C. followed by deaeration and stirring, casting the melt into a mold, and slowly cooling the melt in the mold.

The thus obtained optical glasses of Examples 1-19 were measured for optical constants (refractive index nd and Abbe's number νd), liquidus temperature LT and devitrification resistance. The results are shown in Tables 1 and 2. The prior art glasses (Comparative Examples A-G) were also measured for the same test items, and the results are shown in Table 3. In Table 3, the glass of Comparative Example A conforms to the glass of Example 357 of above mentioned Japanese Pat. Application Kokai (Laid-Open) No. 17409/1974; the glass of Comparative Example B conforms to the glass of Example 7 of above mentioned Japanese Patent Publication No. 40094/1981; and the glasses of Comparative Examples C, D, E, F and G conforms to the glasses of Examples 1, 3, 8, 9 and 10, respectively, of above mentioned Japanese Patent Publication No. 54247/1985.

                                      TABLE 1                                      __________________________________________________________________________                Examples                                                                       1   2   3   4   5   6   7   8   9   10  11                          __________________________________________________________________________     Components (wt. %)                                                             SiO.sub.2   6.0                                                                               18.5                                                                               13.7                                                                                5.0                                                                                5.0                                                                                5.5                                                                               15.5                                                                               15.5                                                                               26.0                                                                               17.5                                                                                9.9                        GeO.sub.2          15.0                                                                                5.0                                                                               18.0                                                                               18.0                                            SiO.sub.2 + GeO.sub.2                                                                      6.0                                                                               18.5                                                                               28.7                                                                               10.0                                                                               23.0                                                                               23.5                                                                               15.5                                                                               15.5                                                                               26.0                                                                               17.5                                                                                9.9                        Na.sub.2 O  8.0                                                                                5.0                                                                               10.0                                                                                8.0                                                                                5.0                                                                                3.0     8.0                                                                               19.0                                                                                7.0                                                                                8.0                        K.sub.2 O  10.0                                                                               12.0                                                                                9.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               18.0                                                                               10.0    10.0                                                                               10.0                        Na.sub.2 O + K.sub.2 O                                                                    18.0                                                                               17.0                                                                               19.0                                                                               18.0                                                                               15.0                                                                               13.0                                                                               18.0                                                                               18.0                                                                               19.0                                                                               17.0                                                                               18.0                        P.sub.2 O.sub.5                                                                           19.1                                                                               14.1                                                                               10.1                                                                               23.8                                                                               17.4                                                                               30.5                                                                               15.4                                                                               14.1                                                                               10.1                                                                               14.8                                                                               17.4                        Nb.sub.2 O.sub.5                                                                          47.5                                                                               30.3                                                                               23.3                                                                               24.8                                                                               29.3                                                                               23.3                                                                               29.5                                                                               29.5                                                                               28.3                                                                               31.3                                                                               21.9                        TiO.sub.2      13.2                                                                               14.0                                                                               11.7                                                                                6.7                                                                                9.7                                                                               14.0                                                                               14.0                                                                               11.7                                                                               12.2                                                                                4.2                        Ta.sub.2 O.sub.5                                   20.0                        ZrO.sub.2                                                                      WO.sub.3                                                                       PbO                                                                            ZnO                                                                            Bi.sub.2 O.sub.3                                                               (x)        47.5                                                                               30.3                                                                               23.3                                                                               24.8                                                                               29.3                                                                               23.3                                                                               29.5                                                                               29.5                                                                               28.3                                                                               31.3                                                                               41.9                        B.sub.2 O.sub.3                                                                            9.4                                                                                6.9                                                                                4.9                                                                               11.7                                                                                8.6     7.6                                                                                6.9                                                                                4.9                                                                                7.2                                                                                8.6                        Al.sub.2 O.sub.3                        2.0                                    As.sub.2 O.sub.3                                                                           0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                                0.2                        nd         1.74655                                                                            1.74876                                                                            1.73902                                                                            1.71717                                                                            1.73114                                                                            1.72015                                                                            1.75283                                                                            1.71371                                                                            1.71585                                                                            1.75280                                                                            1.70910                     νd       26.67                                                                              24.78                                                                              26.77                                                                              26.25                                                                              26.92                                                                              26.48                                                                              24.27                                                                              27.65                                                                              28.49                                                                              24.68                                                                              29.31                      LT (°C.)                                                                           905 1020                                                                               960 940 890 980 1010                                                                               910 1000                                                                               1000                                                                               810                         Devitrification                                                                           Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                      resistance parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                      __________________________________________________________________________      Notes                                                                          1. (x) is the total amount of Nb.sub.2 O.sub.5, Ta.sub.2 O.sub. 5,             ZrO.sub.2, WO.sub.3, PbO, ZnO and Bi.sub.2 O.sub.3.                            2. Devitrification resistance was evaluated by heating and softening 20 g      of a glass block, cooling the softened block to room temperature, and          observing the glass state.                                               

                                      TABLE 2                                      __________________________________________________________________________                Examples                                                                       12  13  14  15  16  17  18  19                                      __________________________________________________________________________     Components (wt. %)                                                             SiO.sub.2   9.9                                                                                9.9                                                                                9.9                                                                                9.9                                                                               9.9  6.5                                                                               11.5                                                                               15.5                                    GeO.sub.2                                                                      SiO.sub.2 + GeO.sub.2                                                                      9.9                                                                                9.9                                                                                9.9                                                                                9.9                                                                               9.9  6.5                                                                               11.5                                                                               15.5                                    Na.sub.2 O  8.0                                                                                8.0                                                                                8.0                                                                                8.0                                                                               8.0  5.5                                                                                7.5                                                                                8.0                                    K.sub.2 O  10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                                                               10.0                                    Na.sub.2 O + K.sub.2 O                                                                    18.0                                                                               18.0                                                                               18.0                                                                               18.0                                                                               18.0                                                                               15.5                                                                               17.5                                                                               18.0                                    P.sub.2 O.sub.5                                                                           17.4                                                                               17.4                                                                               17.4                                                                               17.4                                                                               17.4                                                                               14.8                                                                               14.8                                                                               15.0                                    Nb.sub.2 O.sub.5                                                                          31.9                                                                               21.9                                                                               31.9                                                                               21.9                                                                               36.9                                                                               43.8                                                                               36.8                                                                               34.0                                    TiO.sub.2   4.2                                                                                4.2                                                                                4.2                                                                                4.2                                                                               4.2 12.2                                                                               12.2                                                                               10.2                                    Ta.sub.2 O.sub.5                                                               ZrO.sub.2  10.0                                                                WO.sub.3                   5.0                                                 PbO            20.0                                                            ZnO                10.0                                                        Bi.sub.2 O.sub.3       20.0                                                    (x)        41.9                                                                               41.9                                                                               41.9                                                                               41.9                                                                               41.9                                                                               43.8                                                                               36.8                                                                               34.0                                    B.sub.2 O.sub.3                                                                            8.6                                                                                8.6                                                                                8.6                                                                                8.6                                                                               8.6  7.2                                                                                7.2                                                                                7.3                                    Al.sub.2 O.sub.3                                                               As.sub.2 O.sub.3                                                                           0.2                                                                                0.2                                                                                0.2                                                                                0.2                                                                               0.2  0.2                                                                                0.2                                                                                0.2                                    nd         1.73132                                                                            1.71095                                                                            1.71084                                                                            1.71922                                                                            1.73536                                                                            1.82812                                                                            1.78835                                                                            1.74670                                 νd       28.39                                                                              29.19                                                                              29.39                                                                              28.08                                                                             26.57                                                                               21.90                                                                              23.37                                                                              25.51                                  LT (°C.)                                                                           1020                                                                               795 855 835 850 1020                                                                               1020                                                                               950                                     Devitrification                                                                           Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                                                             Trans-                                  resistance parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                                                             parent                                  __________________________________________________________________________      Note                                                                           For (x) and devitrification resistance, reference is made to the footnote      of Table 1.                                                              

                                      TABLE 3                                      __________________________________________________________________________                Comparative Examples                                                           A     B     C     D     E     F     G                               __________________________________________________________________________     Components (wt. %)                                                             P.sub.2 O.sub.5                                                                           32.3  31.3                                                          SiO.sub.2              10.0  15.0  20.0  20.0  20.0                            Na.sub.2 O 17.7        25.0              20.0  10.0                            K.sub.2 O        16.2        27.0  25.0   5.0  10.0                            BPO.sub.4              22.0  10.0        13.0   2.0                            TiO.sub.2  20.0  24.8  18.0  23.0  30.0  27.0  23.0                            Nb.sub.2 O.sub.5 22.9  25.0  25.0  15.0  15.0  35.0                            WO.sub.3   30.0                                                                ZnO               4.8                                                          Al(PO.sub.3).sub.3                 10.0                                        As.sub.2 O.sub.3        0.5   0.5   0.5   0.5   0.5                            nd         1.8012                                                                               1.8178                                                                               1.7322                                                                               1.7505                                                                               1.7433                                                                               1.7385                                                                               1.8395                          νd      20.8  21.8  26.5  25.5  24.9  26.2  22.5                            LT (°C.)                                                                           >1050 >1050 >1040 >1050 >1050 >1040 >1050                           Devitrification                                                                           Devitrified                                                                          Devitrified                                                                          Devitrified                                                                          Devitrified                                                                          Devitrified                                                                          Devitrified                                                                          Devitrified                     resistance                                                                     __________________________________________________________________________      Note                                                                           For devitrification resistance, reference is made to the footnote (2) of       Table 1.                                                                 

As is clear from Table 1, the glass of Example 5 had a refractive index nd of 1.73114, an Abbe's number νd of 26.92 and a liquidus temperature LT of 890° C. and showed good devitrification resistance, and accordingly was a glass of high refractive index, high dispersion and good stability which had been difficult to obtain in the past. As is also clear from Table 1, the optical glasses of Examples 1-4 and 6-19 as well had a refractive index nd of 1.70 or more and an Abbe's number νd of 30 or less, were free from devitrification and transparent, and showed excellent devitrification resistance. Meanwhile, the glasses of Comparative Examples A-G were all devitrified. In ordinary glasses, when the liquidus temperature LT of a glass is higher, the glass is less stable and is devitrified more easily. Therefore, in Tables 1-3, liquidus temperature LT was shown as a yardstick for devitrification resistance. The optical glasses of Examples 1-19 gave lower liquidus temperatures than the glasses of Comparative Examples A-G, indicating superior devitrification resistance.

As described above, there can be obtained, according to the present invention, an optical glass of high refractive index and high dispersion having a refractive index nd of 1.70 or more and an Abbe's number νd of 30 or less and being excellent in devitrification resistance and stability. Combining this optical glass with an optical glass of low dispersion makes it possible to obtain an achromatic lens having a high aperture ratio. The present invention also enables the mass production of an optical glass of uniformity and free from devitrification, as in the case of production of ordinary optical glasses. Thus, the present invention can be used in production of various optical products and has high practical value. 

What is claimed is:
 1. An optical glass having a refractive index nd of at least 1.70 and an Abbe's number νd of at most 30 consisting essentially, in percent by weight based upon the total composition, of:

    ______________________________________                                         SiO.sub.2            5-30%                                                     GeO.sub.2            0-25%                                                     the sum of SiO.sub.2 and GeO.sub.2                                                                  5-30%                                                     Na.sub.2 O           0-19%                                                     K.sub.2 O            0-19%                                                     the sum of Na.sub.2 O and K.sub.2 O                                                                 10-19%                                                    P.sub.2 O.sub.5      10-35%                                                    Nb.sub.2 O.sub.5     18-49%                                                    TiO.sub.2            0-14%                                                     Ta.sub.2 O.sub.5     0-20%                                                     ZrO.sub.2            0-10%                                                     WO.sub.3             0-5%                                                      PbO                  0-20%                                                     ZnO                  0-10%                                                     Bi.sub.2 O.sub.3     0-20%                                                     the sum of Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5,                                                      18-49%                                                    ZrO.sub.2, WO.sub.3, PbO, ZnO and Bi.sub.2 O.sub.3                             B.sub.2 O.sub.3      0-15%                                                     Al.sub.2 O.sub.3     0-5%                                                      and further comprising                                                         As.sub.2 O.sub.3     0.3% by weight                                                                 based on the                                                                   total amount                                                                   of the above                                                                   components.                                               ______________________________________                                    


2. An optical glass according to claim 4, consisting essentially, in percent by weight based on the total composition of:

    ______________________________________                                         SiO.sub.2           5-26%                                                      GeO.sub.2           0-18%                                                      the sum of SiO.sub.2 and GeO.sub.2                                                                 5-26%                                                      Na.sub.2 O          5-19%                                                      K.sub.2 O           5-18%                                                      the sum of Na.sub.2 O and K.sub.2 O                                                                13-19%                                                     P.sub.2 O.sub.5     10-30%                                                     Nb.sub.2 O.sub.5    22-42%                                                     TiO.sub.2 O         4-14%                                                      Ta.sub.2 O.sub.5    0-15%                                                      ZrO.sub.2           0-5%                                                       WO.sub.3            0-3%                                                       PbO                 0-15%                                                      ZnO                 0-5%                                                       Bi.sub.2 O.sub.3    0-15%                                                      the sum of Nb.sub.2 O.sub.5, Ta.sub.2 O.sub.5, ZrO.sub.2,                                          22-44%                                                     WO.sub.3, PbO, ZnO and Bi.sub.2 O.sub.3                                        B.sub.2 O.sub.3     3-15%                                                      Al.sub.2 O.sub.3    0-3%                                                       and further comprising                                                         As.sub.2 O.sub.3    0-1% by weight                                                                 based on the                                                                   total amount                                                                   of the above                                                                   components.                                                ______________________________________                                    


3. An optical glass according to claim 1, further containing a chromatizing or deaerating amount of Sb₂ O₃, F or both. 